Twins: are they at risk? A longitudinal study of twins and nontwins from birth to 18 years of age.

The purpose of this study has been a comparison of a group of twins (145 paris) and a whole cohort of singletons (114,828 individuals) born 1953 and living in the Stockholm metropolitan area in 1963. The twins and singletons have been followed from birth onwards. A pertinent question is whether twins are at greater risk at birth and whether they are predisposed to below-average mental and physical growth. The results show that twins are more susceptible to lower birth weight, shorter gestation period and birth complications. These factors also seem to have a lasting effect on later physical and mental development. Monozygotic twins, twin girls, and male twins with low birth weight, seem to be particularly at risk for below-average mental and physical growth. An interaction between social background and birth weight can also be seen for the male twin group.